In the prior art, in multi-cylinder dryers of paper machines, twin-wire draw and/or single-wire draw is/are employed. In a twin-wire draw, the groups of drying cylinders include two endless fabrics, belts or wires which press the web, one from above and the other one from below, against the heated cylinder faces. Between the rows of drying cylinders, which are usually horizontal rows, the web has free and unsupported draws. The free draws are susceptible to fluttering which may cause web breaks, in particular since the web is still relatively moist and, therefore, has a relatively low strength. For this reason, in recent years, increasing use has been made of a single-wire draw in which each group of drying cylinders has only one drying wire. The web runs on support of the drying wire through the entire group so that the drying wire presses the web on the drying cylinders against the heated cylinder faces thereof, and whereas, on the reversing cylinders or rolls arranged between the drying cylinders, the web remains at the side of the outside curve. Thus, in a single-wire draw, the drying cylinders are placed outside the drying wire loop and the reversing cylinders or rolls are situated inside the drying wire loop.
A dryer section that consists of normal groups with single-wire draw alone involves the drawback that the paper web is dried from the side of its lower face alone, which may cause a tendency of curling in the web. Therefore, it is known in the prior art, that when drying a paper web, dryer sections are used in which there are, for example, alternatingly so-called normal groups and inverted groups with single-wire draw. In the prior art, dryer sections are also known that consist exclusively of so-called normal groups with single-wire draw. Also, dryer sections are known in which all the other groups except the last group are normal groups with single-wire draw, whereas the last group is a group with twin-wire draw.
In the dryer sections mentioned above, various problems have occurred, for which problems the present invention suggests novel, efficient solutions. These problems include the large length of the dryer section, which increases the costs of the dryer section and the machine hall. Problems have also been encountered in the runnability of the dryer section, in the threading of the web, problems arising from differences in the speeds of different wires, as well as problems related to control of transverse shrinkage of the web. In inverted drying groups, in the event of breaks, a problem consists of the removal of broke, for inverted groups are not self-cleaning by the force of gravity. Generally, these problems tend to become worse as the running speed of the paper machine becomes higher.
In the prior art dryer-section constructions, the overall length of the dryer section becomes considerably large and this increases the cost of investment of a paper mill, above all the construction cost. If it were possible to reduce the length of the dryer section, it would also be possible to reduce the construction cost required by the paper mill in the same proportion.
Regarding the prior art related to the present invention, reference is made to Finnish Laid-open Publication No. 83,246 (applicant-Beloit Corporation, corresponding to U.S. Pat. No. 4,850,121), to German Patent No. 31 90 612 (applicant-J. M. Voith GmbH), as well as to the current assignee's Finnish Patent Application Nos. 913648 (corresponding to U.S. Pat. No. 5,279,050, incorporated by reference herein) and 940992 (corresponding to U.S. Pat. No. 5,539,999, incorporated by reference herein).
Of the prior art cited above, Finnish Patent Application Nos. 913648 and 940992 are most closely related to the present invention. Thus, another object of the present invention is further development of the geometry and the grouping of the cylinders in a group with single-wire draw described in these Finnish patent applications. In these Finnish patent applications, drying cylinders of two different diameters and reversing cylinders of two different diameters are used, of which cylinders the drying cylinders with larger diameter are placed in the uppermost horizontal level, and the reversing cylinders placed in the gaps between every other pair of upper drying cylinders and the drying cylinders of smaller diameter placed in the gaps between every other pair of cylinders are placed in the next, lower horizontal level. The last-mentioned reversing cylinders and drying cylinders have substantially equal diameters. In the lowest, third level, the reversing cylinders of smaller diameter are placed, which are placed in pairs at both sides of the gap between every other pair of upper drying cylinders, a drying cylinder of smaller diameter being placed above, and in the middle of, the gap between the pair of lower reversing cylinders. Drawbacks of the construction described in Finnish Patent Application 913648 include the fact that the drying cylinders and the reversing cylinders are placed at quite a large difference in height in relation to one another, so that servicing of the machine from one tending platform is difficult, and that, for the machine, both drying cylinders of two different diameters and reversing cylinders of two different diameters are needed, which increases the costs of manufacture and spare parts of the dryer section.